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ArcticMyst Security by Avery

Power Supply for a 15W CO2 Laser?

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Oct 8, 2012
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I'd like to build a fairly low power (relitive of course) CO2 laser and found a 15W tube that would appear to fit my bill perfectly. I'm a bit stuck when it comes to power though. The tube can be found HERE. I was thinking to use a neon sign transformer, but I'm not sure what voltage to get one at because of the difference between "Triggering Voltage" and "Operating Voltage" listed in the tube's discription. If I get an 8KV transformer, will it have enough power to get the laser going? It says its trigger voltage is 12KV. Will it overpower the laser and break it? The operating voltage is 5.5KV.
 





phenol

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Oct 30, 2007
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If the NST peak voltage is lower than the specified trigger voltage, then it may not be able to strike the tube. moreover, the nst output should be rectified. to get the current you want, you could put a variac before the nst and a bunch of 10s of kiloohm resistors on tube's anode to cancel out or reduce the inherent neg residtance of the tube. note that these would dissipate some power and should be rated accordingly.
i run my 60w tube at 20w using common crt tv parts-flyback and tripler, some basic pwm controller electronics and a variable bench psu.
 
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So I would get an NST that could hit my trigger voltage, control it with a variac, and protect the trube from higher current with some parellel resistors. Then, when I turn the tube on, I would trun down the variac to the tube's opperating voltage?
 

phenol

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Not parallel, the ballast resistor should be in series with the anode, ideally as close to it as possible. To ignite the discharge, the nst should be able to deliver the required peak voltage. once this happens, the voltage across the tube will drop and the excess power will be wasted as heat in the ballast resistor. Those NST's should be current-limited, but im guessing that this limit is way beyond what this tube can handle. Anyway, hook an ammeter in series with the cathode wire to monitor the current and adjust with the variac.

or better yet buy a switch mode PSU from ebay. it will let you control the current with a simple potentiometer in addition to offering a bunch of safety interlocks, TTL control, etc...
 
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So if I were to use a NST, it should look something like the attached schematic?

EDIT:I've read a few places that the tubes work better on DC and reccomend the use of a full wave reftifer. Thoughts?

With a switch mode PSU, I did an eBay search and didn't come up with much. What I did find appeared to be quite expensive (it would be cheaper to buy a dedicated power unit for a 40W laser in most cases). Maybe I'm not looking right?

I did find an ebay posting for THIS power supply, in the eBay title it claimed it was good for 10, 20, 35, and 40W lasers. I sent an email to LightObject asking about the aplications said power supply is suitable for, hopefully I'll hear from them tomorrow.
 

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phenol

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The ballast needs to be in the kiloohm range. see the picture, the arrangement should be something along those lines, plus a ballast resistor.
read sam's faq for more details and be extremely cautious when dealing with such high voltages.
 

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Awesome, thanks!

I'm going to see what LightObject says about that power supply, but if it's a no-go I'll pick up a NST.

Rep for the help +1
 

phenol

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Where i live we dont really have NSTs growing on trees, so i was forced to approach the situation differently as i couldnt wait for the factory psu that i ordered from China just today.
LightObject seem to offer ZnSe lenses, among other things. If you feel like focusing the laser output down to a small dot, you may consider taking one. If you do, treat it gently and keep it away from anything acidic lest it should release the rather deadly H2Se gas.
 
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What project are you working on?

I've found this NST, and this VARIAC, if LightObject's power supply doesn't work, what are your thoughts on that combo? The NST says it'll put out 12KV, is that cutting it too close (should I go find a 15KV one instead)? The NST imput current is 1.2A max, and the variac is supposed to be good for 3A- unless I missed something it seems like that would work. Always nice to have a second opinion though
 

phenol

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The variac seems adequate, but the NST not so much. First of all, it says that it delivers 12kV p-p, which is only equivalent to an amplitude of 6kV or, assuming a sine wave output, 4.3kV RMS. Second, it loooks like an electronic NST. I dont know if you can adjust its output with a variac, whatever's inside may or may not respond predictably to large input variations. You really need the old NST designs, basically a transformer only w/o any HF electronics involved.
 
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Ah, ok. Well, good news. The power supply in question is good to go, but the employee at LightObject told me I'd need to tone down the power with a resistor, go figure! So, I've put in an order for the tube, PSU, a small motor and tubing for water cooling, and a digital milliammeter. Just have to figure out what size ballast resistor I need and where to find one :) Unless there's anything I missed (I've already got my safety goggles and beam stop)?
 
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You may want to consider looking for a NST with 10 or 15mA rating, since 30mA is too much for the tube, and I don't think a ballast resistor is going to limit the current that much.
 

phenol

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it should be adjustable in a wider range of currents, otherwise a ballast resistor wont do much. i would use parallel and or serial combinations of 5W 100k resistors w/ a properly rated nst, should you choose to go that way. together with the variac you should be able to reach optimum conditions.
 
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It's a constant current supply, phenol. Ballasting it won't do much of anything.

Since it explicitly states "analog power control," it is indeed adjustable.
 

phenol

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That's exactly what i said (meant), as far as the switch mode psu is concerned ;)...and then again it also depends on the compliance range of that brick...
I believe that MrDelsic can omit the ballast altogether if he buys a 40W tube instead and finds a 15-20mA neon-sign xformer.
 




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